Bingowings said:

Hopefully one of the FX savvy guys is reading this and is prepared to mask out the the droid for the few shots you can't just edit him out of.

The next trick would be to create a new shot of Vader and maybe add the Force rumble effect.

Does Carrie scream in fear ever?

She lets out a cry of pain in ROTJ but it's not blood curdling (think Ripley in Aliens when the Alien Queen pulls the power-loader down the hatch to the airlock).

If the the door shut on Leia's cell and we heard a blood curdling scream it would really make Vader look much more nasty.

She screams in fear at the mynock in ESB. Maybe some tweaking could be done to make it sound longer or different? I don't know.